Some engines, known as a variable displacement engines (VDE), may be configured to operate with a variable number of active and deactivated cylinders to increase fuel economy. Therein, a portion of the engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including engine temperature. An engine control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling, and/or through the control of the ignition system to selectively control (e.g., withhold) spark to deactivatable cylinders. By deactivating engine cylinders at low speeds/light loads, associated pumping losses can be minimized, and engine efficiency is increased.
In some instances, the mechanisms that actuate the deactivatable cylinder valves (e.g., VDE mechanisms, VDE actuation systems) may degrade, leaving the intake and/or exhaust valves operating as though the cylinder was still active. In this situation, fuel economy may be impacted as the inability to seal the cylinder during deactivation results in pumping losses. Drivability may also be adversely impacted, as unaccounted air or vapor may be directed through the catalyst from the leaky cylinder. This may be addressed by monitoring VDE mechanism functionality and timely identifying and addressing degradation. Under certain vehicle operating conditions, such as strictly city driving or under heavy loads, the VDE mechanisms may not be exercised regularly, thereby limiting opportunities for diagnosing degradation.
Various approaches have been identified for diagnosing degradation of VDE operation, such as diagnostic methods based on crankshaft vibrations related to engine firing order, firing frequency, measuring manifold pressure, etc. One example approach is shown by Doering et al. in U.S. Pat. No. 8,667,835, where indication of intake and/or exhaust valve degradation is based on an indication of manifold pressure over a plurality of immediately successive induction events of the engine during engine operation. However, the inventors herein have recognized several disadvantages with such approaches. As an example, such approaches may be computationally intensive, requiring a plurality of MAP measurements and extensive data manipulation to perform the VDE system diagnostic while the engine is running. As another example, such approaches may not be able to distinguish between a cylinder with a portion of the cylinder valves functionally degraded and a cylinder with all of the cylinder valves functionally degraded. In yet another example, additional sensors may be required to monitor certain engine parameters in order to diagnose degradation of the VDE mechanisms, leading to increased cost.
Thus, in one example, the above issues may be at least partially addressed by a method including, responsive to a request to diagnose a cylinder valve actuator of an engine during a non-fueling condition of the engine, commanding an EGR valve open and determining a first exhaust gas flow, and deactivating one or more cylinder valves and indicating cylinder valve actuator degradation when a second exhaust gas flow is not less than a threshold relative to the first exhaust gas flow.
In another example, a method for an engine including a first cylinder and a second cylinder includes during non-combustion engine conditions while the engine is rotating, actuating a first intake valve and a first exhaust valve of the first cylinder and measuring a first gas flow rate through an exhaust gas recirculation (EGR) passage coupling an exhaust manifold of the engine to an intake manifold of the engine; deactivating actuation of the first intake valve and first exhaust valve and measuring a second gas flow rate through the EGR passage; and indicating degradation of a variable displacement engine (VDE) system responsive to the first gas flow rate being within a threshold range of the second gas flow rate. In this way, existing exhaust gas recirculation system may be used to assess potential degradation of the VDE mechanisms which reduces the cost associated with adding supplemental diagnostic equipment. The technical effect of evaluating the VDE system during a fuel-off condition with minimal data collection is that diagnostics may be performed independent of an operator's driving habits and without affecting drivability. In particular, by using the existing exhaust gas flow pressure sensor to measure and compare EGR flow rates during VDE and non-VDE modes, potential degradation of the VDE mechanisms may be assessed without extensive computational requirements.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.